Pleuroperitoneum

The pleural and peritoneal membranes, or the membrane lining the body cavity and covering the surface of the inclosed viscera; the peritoneum; -- used especially in the case of those animals in which the body cavity is not divided.

⇒ Peritoneum is now often used in the sense of pleuroperitoneum, the pleurae being regarded as a part of the peritoneum, when the body cavity is undivided.